It is well known to provide an airbag assembly including an inflatable airbag cushion for protecting the occupant of a transportation vehicle. In an automotive vehicle such air bag assemblies are typically located within the hub of the steering wheel and in positions opposing the knees and/or torso of a side passenger. Additional air bag assemblies may be located within the seats and/or door panels for protection of the occupants during a side-impact event.
One particular type of airbag assembly is an inflation driven bolster disposed in opposing relation to an occupant's lower extremities. Such systems typically incorporate an inflatable cushion disposed in an instrument panel behind an outer occupant contact surface in the form of a plate which is covered by a layer of foam with a aesthetic polymeric skin or fabric. Upon activation, the cushion inflates and pushes the plate into contact with the occupant's knees thereby providing early stage dissipation of kinetic energy. By way of example only, and not limitation, one such system is illustrated and described in U.S. Pat. No. 5,931,493 to Sutherland the contents of which are hereby incorporated by reference in their entirety.
As will be appreciated, when an airbag cushion in a knee bolster or other airbag assembly is activated, the pressure is generally proportional to the quantity of inflator gas expelled into the airbag and inversely proportional to the volume occupied by the gas. As the occupant comes into contact with the expanded airbag or outer contact surface, the inflator gas is forced out of the air bag thereby dissipating the kinetic energy of the occupant achieving a so called “ride down” effect. In order to facilitate the discharge of inflator gas from the airbag it is common to incorporate vents in the form of normally open fixed diameter apertures across the walls of the airbag.
In order to provide additional control over the inflation characteristics of the airbag cushion it is known to use tethering elements in the form of straps or webs extending between surfaces of the airbag and anchoring points on structural members. It has also been proposed to use elongate tethers in attached relation to vent closing members to facilitate adjustable venting. Such systems are illustrated and described in U.S. Pat. No. 6,932,385 to Hawthorn et al. the contents of which are hereby incorporated by reference as if fully set forth herein.